Physics lessons with Robbie Buhl

Posted by Iannucci | 5/13/2009 | 5 comments »
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The My Name Is IRL audience is blessed to include people from all walks of life. We delight in the views shared by everyone from budding fans to grizzled race veterans, people all around the world checking out this little slice of website, sharing a singular thing in common (and no, it's not alcohol consumption.) And every once in a while I'm stunned to discover this group includes people who despite reading my constant drivel are otherwise thought to be smart, like say today when I opened the Inbox to find this from "Kenneth".

I do not know how many times over the years I have heard guys like Robbie Buhl proclaim how the increase in relative humidity has caused the air to become more dense.

Unfortunately, nothing could be further from the truth. An increase in humidity (or in ambient temperature, or a decrease in barometric pressure, or an increase in altitude from sea level or any combination of these) will result in a decrease of air density and generally speaking a resultant reduction in performance of power plants (race car engines) along with a reduction of down force and induced drag.

That said, with the proper adjustments to engine fuel-air ratios (less dense air requires a more lean mix of fuel, this is why aircraft get such great fuel economy at altitude) and aerodynamic devices (proper balance of fore and aft wings) on race cars, quite a bit of this can be compensated for. These are the kind of things that Penske racing is famous for and what enables them to obtain that slight edge over their competitors on such a regular basis.

Did you get that? Me neither. As much as I enjoyed the labs I ended up with a "D" in Physics during my one year at Purdue, which may explain why I became the word butcher I am today. At any rate, I busted out my handy dandy Google and went to the Engineering Toolbox to confirm this.

Water vapor - H2O - is composed of one Oxygen atom and two Hydrogen atoms. Hydrogen is the lightest element at 1 atomic unit while Oxygen is 16 atomic units. Thus the water vapor atom has an atomic mass of 18 atomic units. With 18 atomic units water vapor is lighter than diatomic Oxygen with 32 units and diatomic Nitrogen with 28 units.

* Note! Water vapor in air will replace other gases and reduce the total density of the mixture. Dry air is more dense than humid air!
See, now that someone added the bold type and exclamation points, I think I got it. In other words, when Robbie Buhl says that increased humidity means the air is more dense, he's completely and utterly wrong!

I realize Mr Buhl has probably forgotten more about driving than I'll ever know, but this may or may not be contributing to the reasons his Dreyer & Reinbold cars are a bit off this month. I'll leave it to the Einsteins in the audience to decide that.


  1. MDC // May 13, 2009 11:02 AM  

    Someone at the Engineering Toolbox needs to learn the difference between an atom and a molecule. A molecule of water, which consists of two H atoms and one O atom, usually has a weight of 18. But otherwise this seems correct.

  2. A pilot // May 13, 2009 11:25 AM  

    Right. Height, heat, and humidity all work in the same direction - to make air less dense.

  3. SpeedyRB // May 13, 2009 3:16 PM  

    Less Robbie Buhl and more Jon Beekhuis would be great.

    Have you noticed how Robbie Buhl attacks Jon Beekhuis (albeit playfully) at every opportunity?

    Methinks this is because (a) Jon is obviously much smarter so Robbie feels threatened, and (b) Jon comes from the ChampCar side of things...

  4. timnothhelfer // May 13, 2009 5:01 PM  

    If you use the analogy of flying, does the high humidity displace oxygen needed for combustion? Does the humidity add to pressure increasing resistance above 220 MPH?
    Is there a noticeable difference in flying through dry air, fog or clouds? I would be inclined to think the fog or clouds would offer more drag and less horsepower...someone must know.

  5. Mike // May 13, 2009 9:17 PM  

    Speedy, truth be told they're both CART refugees. Both ARS/Indy Lights champions under CART sanction in the late 80's and early 90's. Jon did run in CART's Indycar races for a few seasons with a few teams (including one I worked with) long before the split ever happened. I don't recall Robbie ever getting a shot in a big car in CART, but did in IRL after the split, so ya, there's the diff if any.

    Regarding density, since the fuel adjustments (during a race) are what, 2 or 3 position between the leanest and richest setting...if the density goes down then the mixture will be a tad richer, which is what they always say is the opportunity to make up time on others...though I'd guess that an adjustment on the fuel knob may also make a change in the ignition map to adjust for it, so...what do I know? Density? Ya, there's plenty of it right here in this chair...